Scroll type apparatuses have been well known in the prior art. For example, U.S. Pat. No. 4,824,346 discloses a device including two scrolls each having an end plate and a spiral wrap. The scrolls are maintained angularly offset so that both spiral elements interfit at a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The fluid pockets are defined by the line contacts between the two spiral elements which are interfitted together. One of the scrolls is an orbiting scroll and the other one is a fixed scroll.
The line contacts shift along the surface of the spiral elements by the orbital motion of the scroll to thereby move the fluid pockets to the center of the spiral elements and consequently compress the fluid in the pockets. It is desirable that the sealing force at the line contact be sufficiently maintained in a scroll type compressor. On the other hand, if the contact force between the spiral elements becomes too large in maintaining the sealing line contact, wear to the spiral elements increases. Accordingly, the contact force between the spiral elements must be suitably maintained.
With reference to FIGS. 6(a), 6(b), and 6(c) the operation of this type of compressor is described below.
Three scroll compressor components are shown including disk-shaped rotor 31, crank pin 45, and axial bushing 23. The relative orientations of the centers of disk-shaped rotor 31, crank pin 45, and axial bushing 23 are shown as Os, Od, and Oc, respectively. The distance between Os and Oc is the radius Ro of orbital motion. A line L2 can be defined passing through Oc and Os. Another line L1 can be defined passing through Oc and perpendicular to line L2. When crank pin 45 is fitted into eccentric hole 231 in bushing 23, center Od of crank pin 45 is placed, with respect to OS, on the opposite side of line L1 and also on the opposite side of line L2 in the counterclockwise rotational direction of arrow A of rotor 31. The relative positions of centers Os, Oc and Od is maintained in all rotative positions of rotor 31. Od, at this particular point of motion, is located in the upper left hand quadrant defined by lines L1 and L2.
When rotor 31 rotates, drive force Fd is exerted at Od to the left and reaction force Fr due to the compression of gas appears at Oc to the right, with both forces being parallel to line L1. As the arm Od-Oc swings outwardly by the creation of the moment generated by forces Fd and Fr, the spiral element of the orbiting scroll, which is rotatably disposed on bushing 23 through a needle bearing, is forced toward the spiral element of a fixed scroll. Consequently, the orbiting scroll orbits with the radius Ro around center Os of rotor 31. The rotation of the orbiting scroll is prevented by a rotation preventing mechanism, described in the above patent, whereby the orbiting scroll orbits but keeps its relative angular relationship. The fluid pockets are moved towards the center and thereby compressed by the orbital motion of the orbiting scroll.
When fluid is compressed by the orbital motion of the orbiting scroll, reaction force Fr, caused by the compression of the fluid, acts on the spiral element. This reaction force Fr acts in a direction tangential to the circle of orbiting motion. This reaction force, which is shown as Fr, in the final analysis, acts on center Oc of bushing 23. Since bushing 23 is rotatably supported by crank pin 45, bushing 23 is subject to a rotating moment generated by Fd and Fr with radius E2 (FIG. 6(c)) around center Od of crank pin 45. This moment is defined as Fd(E2)(sin.theta.), where .theta. is the angle between the line Od-Oc and L1, and where Fd=Fr. The orbiting scroll, which is supported by bushing 23, is also subject to the rotating moment with radius E2 around center Od of crank pin 45 and, hence, the rotating moment is also transferred to the spiral element of the orbiting scroll. This moment urges the spiral element of the orbiting scroll against the spiral element of the fixed scroll with an urging or sealing force Fp. Fp acts through a moment arm E3=E2cos.theta.. Since the moments are equal, FpE2cos.theta.=FdE2sin.theta.. Thus, urging force Fp is expressed by the following formula: EQU Fp=Fdtan.theta.
Accordingly, urging force Fp can be controlled by properly choosing the value of the angle .theta.. However, when abnormally high compression of the liquid refrigerant occurs, reaction force Fr increases greater than normal. Consequently, urging force Fp becomes undesirably large. When urging force Fp becomes too large, the contact force between both scroll elements also becomes too large. Thus, abnormal abrasion occurs between the wall surfaces of the scroll elements, thereby deforming and damaging the scroll elements. The problem of abnormal abrasion is further compounded by automotive air conditioning applications in which the scroll compressors are subject to a wide range of rotational speeds. That is, while one predetermined angle .theta. might be sufficient to accomplish the requisite urging force Fp, the urging force Fp becomes excessive when the compressor is operated under higher rotational speeds.